1. Field of the Invention
The present invention relates to a personal watercraft (PWC) which ejects water rearward and planes on a water surface as the resulting reaction. More particularly, the present invention relates to a personal watercraft having a characteristic exhaust system.
2. Description of the Related Art
In recent years, so-called jet-propulsion personal watercraft have been widely used in leisure, sport, rescue activities, and the like. The jet-propulsion personal watercraft is configured to have a water jet pump that pressurizes and accelerates water sucked from a water intake generally provided on a bottom of a hull and ejects it rearward from an outlet port. Thereby, the personal watercraft is propelled.
In the jet-propulsion personal watercraft, a steering nozzle provided behind the outlet port of the water jet pump is swung either to the right or to the left, to change the ejection direction of the water to the right or to the left, thereby turning the watercraft to the right or to the left.
In the personal watercraft, an engine is contained in an engine room located substantially at the center portion of the watercraft in the longitudinal direction and surrounded by a deck and a hull. As the engine, a two-cycle internal combustion engine (two-cycle engine) that is small and lightweight and is capable of generating a high power is employed. Recently, the use of a four-cycle internal combustion engine (four-cycle engine) has been contemplated.
When the four-cycle engine is accommodated in the engine room for the two-cycle engine and is required to have a horsepower as high as that of the two-cycle engine, a horsepower per displacement of the engine needs to be increased in the four-cycle engine.
In the four-cycle engine, it is necessary to extend an exhaust passage in order to obtain the high horsepower. Nevertheless, since the personal watercraft is short in total length and the engine is mounted at the center portion of the watercraft, it is hard to extend the exhaust passage to such an extent that the high horsepower can be obtained in the four-cycle engine.
The present invention addresses the above-described problem, and an object of the present invention is to provide a personal watercraft capable of obtaining a high horsepower when a four-cycle engine is mounted in the personal watercraft limited in total length.
According to a first aspect of the present invention, there is provided a personal watercraft adapted to eject water from an outlet port and to be propelled as the resulting reaction, comprising: a multi-cylinder four-cycle engine contained in an engine room of the watercraft and having even cylinders of four or more cylinders; a water jet pump being driven by the engine, for pressuring and accelerating the water; exhaust passages respectively connected to exhaust ports of the engine; primary collecting exhaust passages each of which is configured to collect two exhaust passages into one passage; and a secondary collecting exhaust passage configured to collect the primary collecting exhaust passages.
In the personal watercraft so constituted, all the exhaust passages can be efficiently collected in the relatively small engine room in such a manner that two exhaust passages are collected into the primary collecting exhaust passage, and then, the primary collecting exhaust passages are collected into the secondary collecting exhaust passage. With such a structure, by utilizing exhaust inertia, exhaust efficiency is increased regardless of the limited length of an exhaust path. In addition, the exhaust system can be simply and efficiently structured. The secondary collecting exhaust passage may be a collecting exhaust passage obtained by extending the primary collecting exhaust passage into one passage, or otherwise a muffler.
Preferably, two exhaust passages connected to cylinders ignited at successive timings may be collected into one primary collecting exhaust passage.
Preferably, an inlet port may be provided at a rear end portion of the exhaust passage or a front end portion of the primary collecting exhaust passage, for introducing water to an inside of the primary collecting exhaust passage. This makes it possible that the temperature of the exhaust gas flowing through the inside of the primary collecting exhaust passage can be lowered. As a result, the actual primary collecting exhaust passage achieves an effect normally provided by a longer primary collecting exhaust passage. This structure is advantageous to the personal watercraft that is short in total length and is configured to have an engine mounted substantially at the center portion of the watercraft.
Preferably, the secondary collecting exhaust passage may be configured to accommodate at least an extended rear end portion of the primary collecting exhaust passage.
In the structure, since the two exhaust passages of the engine are collected into each primary collecting exhaust passage, and at least a rear end portion of the primary collecting exhaust passage is extended to be accommodated in the secondary collecting exhaust passage, the primary collecting exhaust passage can be extended even in the engine room that is limited in space. This structure is advantageous to the personal watercraft that is short in total length and is configured to have the engine mounted substantially at the center portion of the watercraft.
Preferably, the engine may be a four-cylinder engine, the exhaust passage may be an exhaust manifold through which an exhaust gas from the exhaust port of each cylinder flows to the primary collecting exhaust passage, the primary collecting exhaust passage may be comprised of a primary collecting exhaust pipe, and the secondary collecting exhaust passage may be a muffler comprising at least one expansion chamber.
Preferably, two primary collecting exhaust pipes may have semi-circular cross-sections which are joined to have a circular cross-section. This structure is suitable for the relatively small engine room. Further, a penetrating hole through which the joined primary collecting exhaust pipe is introduced into the muffler is a circular hole which is easy to process. Further, when a water jacket of a double-pipe shape is provided at the outer periphery of the primary collecting exhaust pipe to attain a water-cooling structure, an outer casing of the water jacket that is cylindrical and easy to process can be attained.
Preferably, the two primary collecting exhaust pipes may constitute one pipe having two independent passages defined by a separating plate having a curvature in cross-section. The separating plate can absorb thermal distortion caused by thermal change by changing the curvature.
Preferably, the muffler may be a water muffler in which the exhaust gas containing supplied cooling water flows. This offers higher muffling effect in the personal watercraft.
According to a second aspect of the present invention, there is provided a small boat propelled by a multi-cylinder four-cycle engine disposed in an engine room of the boat, comprising: a plurality of exhaust passages respectively connected to exhaust ports of the engine; at least two collecting exhaust passages, each of which is configured to collect the plurality of exhaust passages, the collecting exhaust passages being integral with each other and defined by a separating plate; and an inlet port provided at a rear end portion of the exhaust passage or at a front end portion of the collecting passage, for introducing water to an inside of the collecting exhaust passage.
In accordance with the small boat so constituted, when the primary collecting exhaust passage is comprised of one pipe having two independent passages defined by the separating plate, the exhaust gas and the separating plate which is subjected to increased thermal load are cooled by the water introduced through the inlet port, so that thermal load is reduced. As a matter of course, the present invention is applicable to personal watercrafts which are one type of the small boat.
Preferably, the inlet port may be provided to allow water to be supplied to the separating plate through the inlet port. Thereby, since the separating plate is positively cooled, the heat load acting on the separating plate can be further reduced.
Preferably, the separating plate may be provided so as to form an angle with respect to a horizontal line in a steady state of the small boat. This makes it possible for water to be supplied to the separating plate through the inlet port regardless of the direction in which the inlet port is opened. More preferably, the separating plate is provided vertically.
The above and further objects and features of the invention will be more fully be apparent from the following detailed description with accompanying drawings.